Loose cannons in the solar system

Five years ago today there were a few people who thought it might be all over.

Five years ago today there were a few people who thought it might be all over.

On November 29th, 1996, asteroid Toutatis was due to pass within a mere three million miles of Earth.

Or so it was calculated - but what if the astronomers' predictions, like those of meteorologists from time to time, were not exactly right?

This particular asteroid had been discovered by two French astronomers in 1989.

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They called it after the Gallic god whose name is often invoked in the comic book stories that tell of the adventures of the diminutive Asterix and his large friend Obelix.

Two heroes of ancient Gaul who fear nothing except that some day the sky might fall on their heads.

Toutatis is about three miles long and one mile wide, or about half the size of Valentia Island, Co Kerry.

It has a strange shape, comprising two chunks of rock connected by a narrow neck-like structure.

It tumbles head over heels on its journey around the solar system in an eccentric, four-year orbit that takes it from just inside Earth's orbit out to the main asteroid belt in the vicinity of Mars and Jupiter.

The first approach by Toutatis after its discovery was on December 8th, 1992, when it passed by at about 2.5 million miles from Earth.

Five years ago today it was three million miles away at its nearest point, and last year it missed us by a good six million miles.

But on September 29th, 2004, Toutatis will skim past the Earth at a distance of only 900,000 miles, the closest it has been since 1353 and closer than it will be again until 2562.

But what if it did not miss, and were to hit the Earth?

The best recent example of such an occurrence is the Tunguska event in June 1908, when an asteroid estimated to have been about 50 yards in diameter came to ground near the river of that name in the middle of Siberia.

Trees were levelled radically in a zone extending 30 miles or more from the point of impact, and tremors were recorded on seismographs as far away as London.

It is reckoned that collisions on this scale can be expected every 300 years or so.

On the other hand, impact by an object like Toutatis, a mile or greater in diameter, which would be catastrophic on a global scale, is likely to happen only every half a million years.

And if the astronomers' calculations concerning the orbit of Toutatis are correct, it seems as if our time is not yet come.